Golf balls are formed from a variety of compositions. For example, golf ball covers may be formed from balata, a natural or synthetic trans-polyisoprene rubber, ionomer resins, a durable thermoplastic material, or polyurethanes or polyureas, relatively soft thermoset or thermoplastic materials. Balata covered balls are favored by more highly skilled golfers because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots. However, balata covered balls are easily damaged, and thus lack the durability required by the average golfer.
Ionomer resins have more or less replaced balata as a cover material. Chemically, ionomer resins are a copolymer of an olefin and an α, β-ethylenically-unsaturated carboxylic acid having at least a portion of the acid groups neutralized by a metal ion, as disclosed in U.S. Pat. No. 3,264,272. Commercially available ionomer resins include, but are not limited to, SURLYN® from DuPont de Nemours and Company, and ESCOR® and IOTEK® from Exxon Corporation. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. While these ionomers provide extremely durable covers, however, the spin and feel are inferior compared to balata covered balls.
Polyurethanes have also been recognized as useful materials for golf ball covers since about 1960. Polyurethanes are the reaction product of a polyisocyanate and a polyol cured with a hydroxy-terminated or amine-terminated curing agent. U.S. Pat. Nos. 3,147,324, 4,123,061, and 5,334,673 are directed to methods of making golf balls having a polyurethane cover. The resulting golf balls are durable, while at the same time maintaining the softer “feel” of a balata ball. However, golf ball covers made from polyurethane have not, to date, fully matched SURLYN® golf balls with respect to resilience or the rebound of the golf ball cover, which is a function of the initial velocity of a golf ball after impact with a golf club.
Polyureas have also been proposed as cover materials for golf balls. For instance, U.S. Pat. No. 5,484,870 discloses a polyurea composition formed from the reaction product of an organic isocyanate and an organic amine, each having at least two functional groups. Once these two ingredients are combined, the polyurea is formed, and thus the ability to vary the physical properties of the composition is limited. Like polyurethanes, conventional polyureas are not completely comparable to SURLYN® golf balls with respect to resilience or the rebound or damping behavior of the golf ball cover.
In addition, epoxy resins and acrylate resins have been used in golf ball compositions as compatibilizers. For example, WO 92/12206 discloses a resin composition for golf balls formed from a polyester block copolymer and an ionomer resin, and also including an epoxy-containing compound to improve compatibility between the two polymers. The resultant compositions are purported to have improved delamination resistance, flexibility and modulus of resilience, however, the inclusion of such epoxy-containing copolymers increases the melt viscosity of the resin compositions, which makes the compositions unusuable for certain molding applications. In addition, U.S. Pat. No. 5,321,089 describes a compatibilizers that contains a small amount of acrylate resins to be used in an ionomer-based golf ball cover composition. The disclosed balls had durability properties superior to balata-covered balls, but inferior to golf balls having covers formed from ionomer blends.
As shown above, the majority of conventional compositions for golf balls have advantages and drawbacks when used in a golf ball layer. As such, golf ball manufacturers are continually searching for compositions that deliver an ideal balance for golfers of all skill levels without sacrificing performance benefits, manufacturing efficiencies, or feel.
Interpenetrating polymer networks, or IPNs, are occasionally used to improve key physical properties or to aid in the compatibilization of the components of a polymer mixture or blend. For example, the use of IPNs may assist in improving durability, e.g., improving fracture toughness and microcracking resistance, and thermal and mechanical performance. Various IPNs are discussed in U.S. Pat. Nos. 5,648,432, 5,210,109, and 4,923,934. For example, U.S. Pat. Nos. 4,128,600, 4,247,578, and 4,342,793 disclose IPN technology for plastics based on a two-component urethane system polymerized simultaneously with an acrylic monomer. In addition, U.S. Pat. No. 4,923,934 discloses the formation of an IPN from the reaction of a blocked urethane prepolymer, a polyol, and epoxy resin, and an epoxy-catalyzing agent, such as an anhydride for use in coating applications.
Therefore, it would be advantageous to use the IPN concept to form a composition that capitalizes on the favorable properties, but compensates for deficiencies, of individual polymeric systems typically used for golf ball components. In particular, it would be beneficial to use an IPN that utilizes several polymeric components, compatibilizers, and blocking agents in order to achieve a golf ball composition that maximizes beneficial properties and minimizes potential problems. The present invention provides such compositions.